Periodic trends: oxide acid-base character - Chemistry - HKDSE

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Periodic Trends: A Journey Across Period 3 Oxides

Hey everyone! Welcome to your study notes for one of the coolest topics in Chemistry: Periodic Trends. Today, we're going to focus on the acid-base character of oxides.

"What's an oxide, and why should I care?" Great question! An oxide is simply a compound containing oxygen and at least one other element. By studying the oxides of elements across a period (in our case, Period 3: from Sodium to Chlorine), we can see a beautiful, predictable pattern emerge. Understanding this pattern is like having a chemical superpower – it helps you predict how substances will behave without having to memorize every single reaction!

Don't worry if this seems tricky at first. We'll break it down into small, easy-to-understand pieces. Let's get started!

Part 1: The Cast of Characters - Oxides of Period 3

First, let's meet the elements and their oxides that we'll be investigating. As we move from left to right across Period 3, the elements change from metals to metalloids to non-metals. This change is the key to everything!

Our lineup of oxides is:

Na₂O (Sodium Oxide)
MgO (Magnesium Oxide)
Al₂O₃ (Aluminium Oxide)
SiO₂ (Silicon Dioxide)
P₄O₁₀ (Phosphorus(V) Oxide)
SO₂ (Sulphur Dioxide)
Cl₂O (Dichlorine Monoxide)

(Note: We'll be using these specific formulas as required by the syllabus!)

Part 2: From Giant Lattices to Small Molecules - The Trend in Structure and Bonding

The way an oxide behaves depends heavily on its bonding and structure. Across Period 3, we see a dramatic shift from giant structures to simple, individual molecules.

The Metals: Na, Mg, Al

Sodium Oxide (Na₂O) and Magnesium Oxide (MgO)

These are formed between a metal and oxygen, which have a large difference in electronegativity. This leads to the transfer of electrons, forming a strong ionic bond. These ions are arranged in a regular, repeating pattern called a giant ionic lattice. Think of it like a huge, strong framework of positive and negative ions all holding each other tightly.

Result: This strong attraction makes them hard, crystalline solids with very high melting points.

Aluminium Oxide (Al₂O₃)

Aluminium is on the border between metals and non-metals. So, its oxide has bonding that is mostly ionic but with some covalent character. It still forms a strong giant ionic lattice, which is why it has an incredibly high melting point.

Did you know?

The mineral corundum is a form of aluminium oxide. With tiny impurities, it gives us precious gems like rubies (chromium impurity) and sapphires (iron and titanium impurities)!

The Metalloid: Si

Silicon Dioxide (SiO₂)

Silicon is a metalloid, and it shares electrons with oxygen to form strong covalent bonds. But it doesn't form small, individual molecules. Instead, each silicon atom is bonded to four oxygen atoms, and each oxygen to two silicon atoms, creating a massive 3D network. This is a giant covalent structure.

Result: To melt silicon dioxide (the main component of sand and glass), you have to break these strong covalent bonds, which requires a huge amount of energy. That's why it has a very high melting point.

The Non-Metals: P, S, Cl

Phosphorus(V) Oxide (P₄O₁₀), Sulphur Dioxide (SO₂), Dichlorine Monoxide (Cl₂O)

These are formed between two non-metals. They share electrons to form covalent bonds. Unlike SiO₂, they exist as small, discrete molecules. These are known as simple molecular structures.

Result: The covalent bonds within the molecules are strong, but the forces between the molecules (van der Waals' forces) are weak. It's easy to overcome these weak intermolecular forces, so these oxides have low melting and boiling points. They are often gases, liquids, or low-melting-point solids at room temperature.

⭐ Key Takeaway: Structure Trend

As you move across Period 3, the structure of the oxides changes like this:
Giant Ionic Structure → Giant Covalent Structure → Simple Molecular Structure

Part 3: The Main Event - The Trend in Acid-Base Character

This is the core concept! The bonding and structure we just discussed directly influence whether an oxide will act as an acid, a base, or something in between.

The Left Side: Basic Oxides

The oxides of the metals on the left are basic oxides.

Na₂O and MgO

These ionic oxides contain the oxide ion, O²⁻. When they react with water, this ion readily accepts H⁺ from water, forming hydroxide ions (OH⁻), which makes the solution alkaline (basic).

Reaction with water:

$$Na_2O(s) + H_2O(l) \rightarrow 2NaOH(aq)$$
(Sodium oxide reacts vigorously to form a strong alkali, sodium hydroxide)

$$MgO(s) + H_2O(l) \rightleftharpoons Mg(OH)_2(aq)$$
(Magnesium oxide reacts slowly and is only sparingly soluble, forming a weak alkali, magnesium hydroxide)

As basic oxides, they also react with acids to form a salt and water (a classic neutralisation reaction).

Reaction with acid:

$$MgO(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l)$$

The Middle Ground: An Amphoteric Oxide

In the middle of the period, we find an oxide that can't make up its mind!

Al₂O₃

Aluminium oxide is amphoteric. This is a very important term! It means it can act as both a base and an acid. It's like a diplomat who can negotiate with two opposing sides.

Important: Aluminium oxide is insoluble in water.

Acting as a BASE (reacting with an acid): $$Al_2O_3(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2O(l)$$

Acting as an ACID (reacting with a hot, concentrated alkali): $$Al_2O_3(s) + 2NaOH(aq) + 3H_2O(l) \rightarrow 2Na[Al(OH)_4](aq)$$ (Forms sodium tetrahydroxoaluminate(III))

The Right Side: Acidic Oxides

The covalent oxides of the non-metals on the right are acidic oxides.

SiO₂

Silicon dioxide is acidic, but it's a bit special. Because of its strong giant covalent structure, it is insoluble in water and does not react with it. However, it will react with hot, concentrated alkalis, proving its acidic nature.

Reaction with a base:

$$SiO_2(s) + 2NaOH(aq) \rightarrow Na_2SiO_3(aq) + H_2O(l)$$

P₄O₁₀, SO₂, Cl₂O

These simple molecular oxides all react readily with water to form acids.

Reaction with water:

$$P_4O_{10}(s) + 6H_2O(l) \rightarrow 4H_3PO_4(aq)$$
(Forms phosphoric(V) acid)

$$SO_2(g) + H_2O(l) \rightleftharpoons H_2SO_3(aq)$$
(Forms sulphurous acid)

$$Cl_2O(g) + H_2O(l) \rightarrow 2HOCl(aq)$$
(Forms hypochlorous(I) acid)

As acidic oxides, they also react with bases to form a salt and water.

Reaction with a base:

$$SO_2(g) + 2NaOH(aq) \rightarrow Na_2SO_3(aq) + H_2O(l)$$

⭐ Key Takeaway: Acid-Base Trend

As you move across Period 3, the acid-base character of the oxides changes like this:
Strongly Basic → Basic → Amphoteric → Weakly Acidic → Acidic → Strongly Acidic

Part 4: The Complete Picture and A Handy Summary

Let's put everything together in a simple table. This is your ultimate revision tool for this topic!

Summary of Period 3 Oxides (Na to Cl)

Oxide	Bonding	Structure	Acid-Base Character	Reaction with Water
Na₂O	Ionic	Giant Ionic	Strongly Basic	Forms NaOH (alkali)
MgO	Ionic	Giant Ionic	Basic	Slightly reacts to form Mg(OH)₂
Al₂O₃	Ionic (with covalent character)	Giant Ionic	Amphoteric	Insoluble
SiO₂	Covalent	Giant Covalent	Acidic	Insoluble
P₄O₁₀	Covalent	Simple Molecular	Acidic	Forms H₃PO₄ (acid)
SO₂	Covalent	Simple Molecular	Acidic	Forms H₂SO₃ (acid)
Cl₂O	Covalent	Simple Molecular	Acidic	Forms HOCl (acid)

Memory Aid!

It's simple to remember the main trend:

Metal oxides are generally Basic. (My Basics!)
Non-metal oxides are generally Acidic. (Not Allowed!)
The one on the borderline, Al₂O₃, is special – it's Amphoteric!

Great job making it through! The patterns in the Periodic Table are a fundamental part of chemistry. Keep reviewing the trends for bonding, structure, and acid-base character, and you'll be able to predict chemical properties like a pro. Keep up the great work!